The production of acetic anhydride by contacting in the liquid phase a composition containing methyl acetate and/or dimethyl ether and one or more iodine-containing compounds with carbon monoxide in the presence of a carbonylation catalyst has been reported extensively in the patent literature. See, for example, U.S. Pat. Nos. 3,927,078; 4,046,807; 4,115,444; 4,252,741; 4,374,070; 4,430,273; 4,559,183; 5,003,104; and 5,292,948 and European Patents 8396; 87,869; and 87,870.
In many processes involving coproduction of acetic acid and acetic anhydride, methanol and methyl acetate are fed to a primary carbonylation reactor and products are produced in the presence of CO in one reactor. In such processes, all of the heat of reaction is released in the single reactor. The heat of reaction for methanol carbonylation is quite high, e.g., about twice that of the heat of reaction for methyl acetate carbonylation. Thus, a large amount of heat must be removed from the primary carbonylation reactor under severe conditions of high pressure and a corrosive environment. Furthermore, the reactions that form acetic acid and acetic anhydride occur simultaneously, leaving no opportunity to sequence the reactions in such a way as to allow the exothermic reactions to occur separately. Methods of sequencing the reaction are thus desirable.